1. Field of the Invention
The present invention relates to a method for producing an optical waveguide easily and inexpensively and a composition of material adapted for the method. The present invention also relates to a self-written optical waveguide having a columnar core made of the cured of a photo-curable resin solution. The term “photo-curable resin solution” used in the invention means a mixture of a photopolymerization initiator and at least one of photopolymerizable monomer, photopolymerizable macro-monomer and photopolymerizable oligomer.
2. Description of the Related Art
Attention has been paid to a technique of leading a light beam with a predetermined wavelength into a photo-curable resin solution to thereby use a self-focusing phenomenon to form an optical waveguide device. For example, a method for producing an optical waveguide has been described in each of JP-A-2000-347043 and JP-A-2002-169038 by joint applicants of the invention. On the other hand, there is known a technique described in each of JP-A-2002-31733 and JP-A-2002-258095 by other applicants than the joint applicants of the invention. In each of these proposed techniques, a core with a high refractive index is formed while all the outside for covering the core is formed as a clad with a low refractive index. On this occasion, after a photo-curable resin solution with a high refractive index is cured to form a core, a photo-curable resin solution with a low refractive index provided in the outside of the core or a mixture solution containing a photo-curable resin solution with a high refractive index and a photo-curable resin solution with a low refractive index provided in the outside of the core is cured.
On the other hand, according to Japanese Patent Application No. 2002-313421 by the joint applicants of the invention, there has been proposed a method for producing an optical waveguide by using a mixture solution containing a photo-curable resin solution with a high refractive index and a photo-curable resin solution with a low refractive index, comprising the steps of: curing the low-refractive-index photo-curable resin solution selectively while enveloping the high-refractive-index photo-curable resin solution in the low-refractive-index photo-curable resin solution; attaching the cured of the low-refractive-index photo-curable resin solution to an outer circumferential portion by leakage light rays; and curing all uncured part of the mixture solution. This optical waveguide producing method is quite different from the techniques described in JP-A-2000-347043, JP-A-2002-169038, JP-A-2002-31733 and JP-A-2002-258095. That is, this is a method for producing an optical waveguide including a high-refractive-index optical path portion as the cured of the mixture solution, a low-refractive-index portion rich in low-refractive-index resin component formed on the outer circumference of the optical path portion, and a high-refractive-index portion further formed as the cured of the mixture solution on the outside of the low-refractive-index portion.
In the related art according to JP-A-2000-347043, JP-A-2002-169038, JP-A-2002-31733 and JP-A-2002-258095, that is, in the technique of curing the high-refractive-index photo-curable resin solution to form a core and then curing the mixture solution of the high-refractive-index photo-curable resin solution and the low-refractive-index photo-curable resin on the outside of the core, it is necessary to selectively cure the high-refractive-index photo-curable resin solution. On this occasion, the maximum core-forming speed is limited to a predetermined value if the high-refractive-index photo-curable resin solution in the mixture solution of the high-refractive-index photo-curable resin solution and the low-refractive-index photo-curable resin solution is selectively cured to form a core so that the low-refractive-index photo-curable resin solution is not contained in the core as perfectly as possible, that is, the low-refractive-index photo-curable resin solution is diffused to the outside as sufficiently as possible. Moreover, if a core is formed from only the high-refractive-index photo-curable resin solution and then a clad is formed on the outside of the core after cleaning, the time required for production of the optical waveguide becomes longer.
Incidentally, optical waveguides have been described in JP-A-8-320422 and JP-A-11-326660. In each of techniques disclosed in JP-A-8-320422 and JP-A-11-326660, a light beam is applied on a photo-curable resin solution through an optical fiber or the like so that a columnar cured resin is formed by a self-focusing phenomenon. In each of the optical waveguides, the columnar cured resin is used as a core. In these techniques, it is necessary to remove uncured part of the photo-curable resin solution from the periphery of the core. As measures to remove uncured part of the photo-curable resin solution, use of a solvent such as toluene has been proposed. On the other hand, the present inventors have developed a technique disclosed in JP-A-2000-347043 and have gotten a patent (Japanese Patent No. 3,444,352). This technique has disclosed conditions adapted to the case where a light beam used for curing a photo-curable resin solution is led into the photo-curable resin solution through an optical fiber so that a columnar cured resin having a diameter nearly equal to the diameter of a core portion of the optical fiber is formed with an end of the optical fiber as a start point. When uncured part of the photo-curable resin solution is removed and the circumference of cured part of the photo-curable resin solution is surrounded by a resin having a refractive index lower than the refractive index of the cured part of the photo-curable resin solution, a module etc. containing the cured resin formed previously and having a higher refractive index as an optical waveguide can be formed easily. Incidentally, when a light beam capable of curing only a high-refractive-index photo-curable resin solution is led into a mixture solution containing the high-refractive-index photo-curable resin solution and a low-refractive-index photo-curable resin solution through an optical fiber, a columnar cured resin can be formed more easily. This technique has been also disclosed in JP-A-2000-347043.
When a high-refractive-index photo-curable resin solution which is not a mixture solution of a high-refractive-index photo-curable resin solution and a low-refractive-index resin solution is used for forming a columnar cured resin, it is necessary to fill a vessel etc. with another photo-curable resin, for example, having a lower refractive index after uncured part of the high-refractive-index photo-curable resin solution is removed from the vessel as perfectly as possible. If uncured part of the high-refractive-index photo-curable resin solution remains on the outer circumference of the core, the shape of the core will change from a designed shape at a connection portion between the vessel etc. and the core when uncured part of the high-refractive-index photo-curable resin solution is entirely cured because uncured part of the high-refractive-index photo-curable resin solution remains particularly in the connection portion.
More specifically, when a self-written optical waveguide needs to be formed in combination with a total reflection mirror, a half mirror or a dichroic mirror as shown in FIG. 10A, there is a possibility that the shape at a connection portion between the core 105 and the total reflection mirror 107, the half mirror 106 or the dichroic mirror will change totally from a designed shape as shown in FIG. 10B unless uncured part 111 of the high-refractive-index photo-curable resin solution is removed perfectly after the core 105 is formed. It is however not easy to remove uncured part of the photo-curable resin solution perfectly because the viscosity of the photo-curable resin solution is generally so high that uncured part of the photo-curable resin solution is deposited on the cured core portion by surface tension. If the removal of uncured part of the photo-curable resin solution is imperfect, the following problem will occur.
First, if uncured part of the photo-curable resin solution is cured after a material for forming a clad is embedded while the uncured part of the photo-curable resin solution is deposited on the core portion, the outer surface of the core becomes so bumpy that a scattering loss increases. In a complex portion such as a branch portion, a redundant loss such as a branch loss increases. Incidentally, the possibility that uncured part of the photo-curable resin solution will remain in the branch portion is a significant issue even in the case where the viscosity of the photo-curable resin solution is low. Even in the case where uncured part of the photo-curable resin solution is not deposited on the core, uncured part of the photo-curable resin solution may cause scattering of stray light or peeling of the clad from a housing or another optical component if the uncured part of the photo-curable resin solution remains in the inside of the housing or on the optical component. As a result, the uncured part of the photo-curable resin solution may exert not a little bad influence on communication characteristic so that the influence leads to lowering of reliability.
Incidentally, use of a solvent may exert the following bad influence. First, the process of filling the vessel with a solvent, cleaning the vessel and drying the vessel increases. The characteristic of the core member deteriorates because the core member may be swollen or deformed by the solvent. The respective optical components, the core and the housing are destroyed easily because of peeling when the solvent penetrates into the interface between any two of the respective optical components, the core and the housing. Penetration of the solvent into the core makes optical characteristic deteriorate.